"Prima la musica e poi le parole" or first the music and then the words.  This is the title of an opera by Salieri, who in the play and film Amadeus was supposed to have killed Mozart.  Of course the music is important, but if the words in a song didn't matter, the composer would simply have written "la la la".  The words do matter, and it's our responsibility as singers to put them across well, whether they're in our own language, or one we don't even speak.

Many people think it's just a question of "spitting out the consonants", but it's actually the vowels that carry the singing sound.  If you only focus on the consonants, all we'll get is a lot spitting and not much singing.

Try singing a phrase of a song to one smooth, open vowel, say a nice clear "ah". Try again, on an "ooh".  Easy?  OK, now try on an "eeh".  A bit harder?  That's because "eeh" is a 'closed' vowel.  Try to make sure your lower jaw is more relaxed than when speaking - no Wallace and Gromit-style cheesy grins!  Now for the hard bit - sing the melody to the words, but missing out all the consonants.  Once you've got your head (and mouth) around this, you should get a long, continuous line of changing vowel sounds.  Try it a couple of times until it feels a bit more natural.  Watch yourself in the mirror - remember, you want to look like you when you're singing, not someone pulling faces.

Once you've got the hang of this, put the consonants back in, but keep them very light, on the tip of your tongue.  The muscular memory that's been developing in the last few minutes is of a lovely, smooth line of sound, uninterrupted by all those pesky consonants.  Your brain will be sending a message to your mouth to keep that smooth line of sound going.

Now you're singing clear words, but also a really legato (smooth) line of melody.